Project Summary Wilms tumor is the most common pediatric kidney cancer and the 3rd most common solid tumor of childhood. Wilms tumor is treated with a combination of surgery, chemotherapy, and radiation, and while most children are cured, survival remains poor in those with advanced-stage disease. Adverse late effects of chemotherapy are common and affect the quality of life of survivors. Better molecular knowledge of Wilms tumor is necessary to develop novel therapies that are more effective and less toxic. Known driver mutations (WT1, WTX, and CTNNB1) are identified in only one-third of Wilms tumors. Recently, we sequenced a large cohort of Wilms tumor specimens and identified recurrent, somatic heterozygous missense mutations in the enzyme DROSHA, which were mutually exclusive with known driver mutations in WT1 and CTNNB1. DROSHA is a ribonuclease that is essential for the first step in the biogenesis of microRNAs (miRNAs), small RNAs that play critical roles in the biology of cancer. Wilms tumor is the first human cancer in which DROSHA mutations have been identified. However, it is currently unknown how DROSHA mutations impact miRNA biogenesis and contribute to Wilms tumor pathogenesis. DROSHA mutations in Wilms tumors occur at or near conserved metal-binding residues in the ribonuclease (RNase) III domains, and impair miRNA processing. Importantly, we discovered that Wilms tumors with DROSHA mutations exhibit greatly reduced expression of a specific sub-class of miRNAs, including multiple members of the let-7 tumor suppressor miRNA family. let-7 miRNAs regulate several known oncogenes in Wilms tumors, including MCYN and LIN28, and therefore loss of expression of these miRNAs is likely to contribute to tumorigenesis in this setting. To establish the clinical significance of molecular subtype in Wilms tumors, we will correlate the presence of known and novel mutations with clinical outcome in a large collection of clinically-annotated Wilms tumor specimens. Our preliminary data suggest that heterozygous DROSHA mutations operate through a dominant-negative mechanism. We hypothesize that heterozygous DROSHA RNase III missense mutations drive tumorigenesis by impairing the biogenesis of a specific set of tumor suppressor miRNAs, reprogramming the miRNA expression program in the developing kidney to facilitate Wilms tumor development. We will test this hypothesis using state-of-the-art genomic editing techniques to recreate tumor-specific DROSHA mutations in Wilms tumor cells. We will identify and validate specific miRNAs whose dysregulated expression drives tumorigenesis. We will test a novel targeted therapeutic strategy using genetically-engineered cell lines and mouse models of Wilms tumor. Elucidation of these mechanisms will provide critical molecular insights into Wilms tumor and potentially reveal new therapeutic approaches based on miRNA delivery or pharmacologic modulation of downstream pathways.